The present invention relates to a semiconductor laser device and a semiconductor laser module provided with a semiconductor laser element for outputting a laser beam having a plurality of oscillation longitudinal modes.
The recent and rapid spread of the Internet and sudden increase of connection between in-company LANs, has resulted in an increase of the number of communication calls and in an increase in data traffic. This increase in traffic has stressed current optical systems. To prevent the communication performance from deteriorating, the use wavelength division multiplexing (WDM) technologies has advanced and spread.
WDM systems support transmission volumes 100 times larger than the capacity of conventional fiber optic communications by superimposing a plurality of optical signals at different wavelengths on a single fiber. Current WDM systems are capable of long distance transmissions by performing optical amplification with an erbium-doped fiber amplifier (EDFA) or Raman amplifier. An EDFA is an optical fiber amplifier with erbium added. When light having a wavelength of a 1,550 nm band and serving as a transmission signal passes through the EDFA, an additional light emitted by an exciting laser with a wavelength of 1,480 nm or 980 nm is introduced to amplify the signal.
A Raman amplifier is an amplifier capable of directly amplifying signal light by using an already laid optical fiber as an amplifying medium and introducing amplifying light via using stimulated Raman scattering.
Typically for long distance optical transmission with a WDM system, the interval between repeaters must be small due to inefficient amplification. With more repeaters, costs increase. Therefore, to be able to increase the interval between repeaters, one can either increase the output of a semiconductor laser device used for a signal light source and/or improve the amplifying capacity of the repeater.
To meet the above requirements, a semiconductor laser element capable of outputting a laser beam of 250 mW or more is used for EDFA excitation. This higher power level requires high reliability from the semiconductor laser devices.
In the case of a WDM system it is especially important to maintain highly accurate oscillation control and high output operation, not only for the signal light source, but also for the exciting light source used for amplification. The heat of a semiconductor laser element produced due to current injection is known to be a large factor for degrading the oscillation control and high output operation. To compensate for this thermal degradation problem, various conventional approaches are used.
For example, in the case of a conventional semiconductor laser device, a thermistor for measuring the temperature of a semiconductor laser element is often set nearby the semiconductor laser element so that the temperature of the semiconductor laser element can be controlled by an electrothermal element such as a Peltier element.
FIG. 18 is a front view showing a schematic configuration of a conventional semiconductor laser device. In FIG. 18, a submount 102 formed by AlN having an insulating property and a high heat conductivity is set on a carrier 101 formed by CuW. A semiconductor laser element 103 for outputting a laser beam of a predetermined wavelength is set on the submount 102. A submount 104 formed by AlN is set on the carrier 101 and, a thermistor 105 for measuring the temperature of the semiconductor laser element 103 is set on the submount 104.
The semiconductor laser element 103 and the submount 102 are joined to each other through a metallic thin film 102a. The metallic thin film 102a contains layers of Ti, Pt, and Au at thicknesses of 60 nm, 200 nm, and 600 nm respectively. The semiconductor laser element 103 and submount 102 are joined on the metallic thin film 102 by a solder material such as AuSn. The thermistor 105 and submount 104 are also joined through a metallic thin film 104a. 
The face of the semiconductor laser element 103 to be joined with the submount 102 serves as a p-side electrode and the upper face serves as an n-side electrode. The semiconductor laser element 103 is set so that the active layer serving as a main heat generating source is present at the p-side electrode side and located nearby the submount 102. The n-side electrode is connected to a negative electrode by an Au wire 106a. The p-side electrode is connected to the carrier 101 at the positive electrode side through the metallic thin film 102a and an Au wire 106b. 
Submount 102 secures the insulation of the semiconductor laser element 103 and functions as a heat sink of the semiconductor laser element 103. In the case of the carrier 101, as illustrated, the bottom is joined to a CuW base 110 by AuSn solder. The base 110 is set on a Peltier element 120. The Peltier element 120 is controlled by a temperature control section (not illustrated) correspondingly to the temperature detected by the thermistor 105. As a result, the temperature of the semiconductor laser element 103 is controlled by the thermistor 105, Peltier element 120, and the temperature control section.
The thermistor 105 is also insulated from the carrier 101 by the submount 104 to detect the temperature of the semiconductor laser element 103 through the submount 102, carrier 101, and submount 104, each of which has a high heat conductivity.
The heat generated in the semiconductor laser element 103 is conducted to the thermistor 105 through the metallic thin film 102a, submount 102, metallic thin film 102b, carrier 101, metallic thin film 104b, submount 104, and metallic thin film 104a in order. As recognized by the present inventors the heat conducting distance degrades overall operations as the actual temperature detection of the semiconductor laser element 103 is delayed. Moreover, the generated heat passes through the metallic thin films 102a, 102b, 104b, and 104a that are joined to each other by four AuSn solder joints. However, because the AuSn solder joints are used for the junction they respectively have a large heat joints resistance, and so the heat resistance of the above heat conducting path is increased. Moreover, because of the deterioration of the temperature detection accuracy, the temperature control accuracy is also deteriorated. Thus, in conventional WDM applications, the oscillation wavelength of the semiconductor laser element 103 is prone to becoming unstable due to heat generated at high powers, and the system optical output and service life are deteriorated.
Another limitation is present in conventional WDM applications. When supplying current to a semiconductor laser element in order to obtain a high optical output, a voltage drop of an Au thin film in the metallic thin film 102a occurs. Assuming the total resistance of the Au thin film as 0.12 xcexa9, and an inter electrode voltage of a semiconductor laser element when a current of 1 A circulates through the semiconductor laser element equal to approximately 2V, the voltage drop of the semiconductor laser element in the resonator length direction becomes non-uniform by 0.12 V. This leads to the current injection to the semiconductor laser element to become non-uniform and the light density in the active layer also to become non-uniform. The present inventors have discovered this to accelerate deterioration of a device""s optical output and service life.
All of the above described limitations are more pronounced in a semiconductor laser element that includes a diffraction grating. Examples of such laser elements include those disclosed in Japanese Patent Application No. 2000-323118, Japanese Patent Application No. 2001-134545, and Japanese Patent Application No. 2002-228669 filed on Oct. 28, 2000, May 1, 2001 and Jul. 27, 2001 respectively in the Japanese Patent Office, the entire contents of which being incorporated herein by reference. These types of laser elements are configured to output a laser beam with a plurality of oscillation longitudinal modes at or below a threshold at which stimulated Brillouin scattering occurs. These types of laser elements suffer unique degradations in performance due to temperature effects. Specifically, the temperature of a semiconductor laser element rises with increases in the current injected into the laser element""s active layer. This temperature increase causes the refractive index of the diffraction grating layer to change. This change in refractive index causes the selection wavelength of the diffraction grating to shift such that the desired central output wavelength cannot be obtained. Therefore, in the case of an iGM laser that has a diffraction grating, the present inventors determined that it is desirable to go beyond conventional measures to accommodate the heat producing high powers.
This invention provides a semiconductor iGM laser device and a semiconductor iGM laser module capable of preventing the optical output and service life of a high output semiconductor laser element from deteriorating the way they do with conventional device. This is achieved by using new materials in such devices and modules to enable a reduction in the number of interfaces between a laser element and a temperature measuring device, as well as a reduction in the number of interfaces between a laser element and a Peltier module. In addition, performance is enhanced by improving the uniformity of voltage drop of the semiconductor laser element in the resonator length direction by distributing driving currents along a length of the resonator cavity.
The first configuration of the present invention includes a semiconductor laser device having a first mount, a second mount formed by a heat sink having a heat conductivity of 500 W/(m*K) or more and set onto the first mount, and a semiconductor laser element joined on to the second mount through a multi-layer film including a gold thin film. The semiconductor laser device also includes a thermistor set on a third mount which is joined onto the first mount through a multi-layer film, including a gold thin film.
The second configuration of the present invention includes a semiconductor laser device having a first mount, a second mount formed by a heat sink having a heat conductivity of 500 W/(m*K) or more and set onto the first mount, and a semiconductor laser element joined on to the second mount through a multi-layer film, including a gold thin film. The semiconductor laser device also includes a thermistor joined directly onto the first mount through a multi-layer film, including a gold thin film.
The third configuration of the present invention includes a semiconductor laser device having a first mount, a second mount formed by a heat sink having a heat conductivity of 500 W/(m*K) or more and set onto the first mount, and a semiconductor laser element joined on to the second mount through a multi-layer film, including a gold thin film. The semiconductor laser device also includes a thermistor also joined onto the second mount through a multi-layer film, including a gold thin film.
A fourth configuration of the present invention includes a semiconductor laser module having the semiconductor laser device according to any one of the first to third aspects, a temperature controlling element for controlling the temperature of the above semiconductor laser element in accordance with the temperature output from the above temperature measuring element, and a fourth mount set onto the above temperature controlling element, in which the above semiconductor laser device is set onto the fourth mount and the above semiconductor laser element is temperature controlled through the fourth mount.